Method for extracting heat from an effluent flowing in a duct, heat exchanger and system for carrying out such a method

ABSTRACT

Method for extracting heat from an effluent flowing in a duct ( 2 ), in particular a wastewater collection device. In said method, a heat exchanger (E) that is submerged in the effluent is mounted at least on the bottom of the duct, the heat exchanger (E) being formed by coating tubes ( 3 ) with a material that is sufficiently heat-conducting, is poured around the tubes, and hardens in situ. A heat transfer fluid flows in the tubes ( 3 ), the heat being transferred from the effluent in the duct through the molded coating, the upper surface of the poured material being in direct contact with the effluent flowing in the duct, but not with any inserted mechanical part.; The coating is composed of multiple layers having different material properties, i.e. a layer ( 9 ) of heat insulation material between the tubes ( 3 ) and the wall of the duct ( 2 ), a layer ( 10 ) of heat conducting material between the tubes and the effluent, in contact with the tubes, and an abrasion-resistant layer ( 11 ) on the surface, in contact with the effluent.

The invention relates to a method for extracting heat from an effluent flowing in a duct, in particular for extracting heat from a mains sewer, a method according to which a heat exchanger that is immersed in the effluent is installed at least in the bottom of the duct.

Main sewers or ducts for evacuating wastewater transport dirty waters that are tepid or temperate because of their residential or tertiary provenance, or because of their provenance from collective or industrial activities: abattoirs, sports and leisure infrastructures, swimming pools, gymnasia, etc.

The noticeable heat of these waters represents a source of energy that can with advantage be recovered for purposes of heating buildings, of producing hot domestic supply water, or any other thermal use, in combination with a heat pump.

DE 197 19 311 C5 shows a heat exchanger used to implement a method for extracting heat from an effluent. The elements of the heat exchanger may be in the form of metal plates closely following the bottom of the duct or in the form of U-shaped metal section pieces that are connected to pipes for the inflow and outflow of heat-transferring fluid. The heat exchangers are made essentially of metal parts, notably of stainless steel. Such exchangers are relatively costly to manufacture and use. These contact exchangers have their surface placed parallel to the flow of the effluent flowing in the duct. There is no obstacle on the route followed by the effluent and the risk of clogging the surface of the exchanger is reduced. However, a residual deposit may be produced locally between the sections.

This method of heat exchange with an effluent flowing in a duct requires a relatively large, open-out length of contact because of the relatively small surface area in contact, and because of the relatively low temperature of the effluent.

The heat exchanger is an item of equipment fitted to the inside of the ducts; even though these ducts are of large dimension, installation is complicated because of the hydraulic connections that absolutely must be watertight and because of the shape of the parts of the exchanger that must match the internal profile of the ducts.

The object of the invention above all is to provide a method making it possible to effectively extract heat from an effluent flowing in a duct, notably a main sewer, the implementation of which is rapid, simple and economical. It is desirable that the installations for applying the method require only reduced maintenance for an improved service life.

According to the invention, a method for extracting heat from an effluent flowing in a duct, notably a main sewer, according to which a heat exchanger is installed at least in the bottom of the duct, which heat exchanger is immersed in the effluent, the heat exchanger being formed by coating of tubes with a sufficiently heat-conducting material poured around the tubes, and suitable for hardening in situ, the tubes being designed for the flow of a heat-transferring fluid, the exchange of heat with the effluent of the duct taking place through the molded coating, the top surface of the poured material coming into direct contact with the effluent flowing in the duct, is characterized in that the coating is carried out in several layers of materials with different properties, namely:

-   -   a layer of thermally insulating material between the tubes and         the wall of the duct,     -   a layer of thermally conducting material in contact with the         tubes, between the tubes and the effluent,     -   and, on the surface, in contact with the effluent, a layer of         abrasion-resistant material.

Advantageously, after having diverted or temporarily stopped the flow of effluent in the pipe, the exchanger is implemented directly inside the duct

-   -   by depositing, in the bottom of the duct, tubes for a circuit of         heat-transferring fluid,     -   and by pouring in place the coating material in order to immerse         the tubes therein, the material closely matching the profile of         the bottom portion of the duct.

The coating material may consist of at least one of the following materials: synthetic resin, thermally conductive cement or concrete, composite material laden with thermally conductive additions or additives.

It is possible to insert a protective film between the inner surface of the duct and the exchanger incorporated into this duct.

Preferably, the tubes are placed parallel to the longitudinal direction of the duct. The tubes, before pouring of the coating material, may be kept parallel and spaced apart by a rack.

An intermediate webbing, made of metal or of synthetic fibers, may be deployed before pouring of the coating material over at least a portion of a cluster of tubes in order to enhance the mechanical strength and/or improve the thermal transfer.

Advantageously, the tubes are flexible or semi-rigid in order to be able to be rolled up and unrolled continuously over a considerable length, notably of several tens of meters. The tubes may be made of flexible metal material or of synthetic material (polypropylene, plastic). It is possible to prefabricate clusters of flexible tubes and to roll them up on reel-shaped supports for subsequent placement in the effluent duct by unrolling from the reel.

Tubes for heat-transferring fluid may be placed along the complete contour of the cross section of the duct, and coating of the tubes is carried out over the whole contour of the section of the duct.

The internal surface of the molded material may be horizontal, or in an arc of concave curvature toward the inside.

The invention also relates to an exchanger for extracting heat from an effluent flowing in a duct, notably a main sewer, designed to be installed at least in the bottom of the duct and to be immersed in the effluent, characterized in that it consists of tubes embedded in a sufficiently heat-conducting material poured around the tubes that are intended for the flow of a heat-transferring fluid,

the exchange of heat with the effluent of the duct taking place through the molded coating, to the exclusion of any fitted mechanical part, the upper surface of the poured material coming directly into contact with the effluent flowing in the duct, the exchanger being produced according to a method as defined above.

The heat exchanger immersed in the effluent comprises at least five parallel tubes in a regular angular distribution.

The invention also relates to an installation for extracting heat in an effluent duct, notably in a main sewer, characterized in that it comprises in the bottom, immersed in the effluent, at least one heat exchanger made according to the method defined above.

The installation may comprise, in the areas of the internal wall of the duct that are situated above the effluent, pipes exposed to the atmosphere prevailing in the duct, through which pipes a heat-transferring fluid travels in order to recover a portion of the noticeable heat and of the latent heat from condensation of the water vapor.

The invention thus consists in producing directly inside the duct a one-piece, superficial-contact, parallel-streams exchanger consisting of tubes that are embedded in a material poured in place, which closely matches the profile of the bottom portion of the duct.

The heat exchange takes place through the molded wall and not by means of a fitted, dished mechanical part.

Apart from the arrangements explained above, the invention consists of a certain number of other arrangements which will be dealt with more explicitly below with respect to exemplary embodiments described with reference to the appended drawings but which are in no way limiting. In these drawings:

FIG. 1 is a vertical cross section of a wastewater duct in which a heat exchanger according to the invention is installed in the bottom portion.

FIG. 2 is a partial vertical cross section, on a larger scale, of a variant embodiment of an exchanger according to the invention.

FIG. 3 is a vertical cross section of a cluster of parallel tubes surmounted by a webbing, for the production of an exchanger according to the invention.

FIG. 4 is a vertical cross section, similar to FIG. 1, of a duct with a groove in the bottom portion for receiving the molded exchanger.

FIG. 5 is a vertical cross section, on a smaller scale, similar to FIG. 4, showing an exchanger in the form of a meniscus.

FIG. 6 is a vertical cross section similar to FIG. 5 with exchanger tubes distributed along the whole periphery of the internal section of the duct, with wall molded over the whole of this periphery.

FIG. 7 is a diagram in cross section, on a larger scale, of two clusters of tubes and of yoke-shaped supports, in open position for one cluster and closed for the other cluster.

FIG. 8 is a partial view of an annulated tube that can serve for the exchanger.

FIG. 9 is a cross section of an installation with exchanger in the bottom portion and pipes for heat-transferring fluid in the top portion, and

FIGS. 10-12 are cross sections of different profiles of pipes for heat-transferring fluid with fins.

With reference to the drawings, notably to FIG. 1, it can be seen that, to extract heat from an effluent 1 flowing in a duct 2, more particularly a main sewer, a heat exchanger E that is immersed in the effluent 1 is installed at least in the bottom of the duct. The presumed maximum level la of the effluent is represented by a dashed line.

This exchanger E consists of flexible or semi-rigid tubes 3, essentially parallel to the longitudinal direction of the duct. The tubes 3 are placed in clusters and are coated with a sufficiently heat-conducting material 4 poured around the tubes and capable of hardening. The tubes 3 are intended for the flow of a heat-transferring fluid, and the heat exchange with the effluent 1 of the duct takes place through the molded coating, to the exclusion of any fitted mechanical part. The top surface 5 of the poured material comes into direct contact with the effluent 1 flowing in the duct.

The coating material 4 may consist of a synthetic resin, or of a cement or a concrete that is thermally conductive, or of a composite material, optionally laden with thermally conductive additions or additives.

The silicon carbide-based conductive cements or bricks, used in furnaces, are relatively costly. Cements laden with ferritic carbon steel or with metal needles are well suited to the production of the exchanger according to the invention.

The coating material 4, in particular a laden cement, is notably chosen to have a thermal conductivity greater than 5 W/m.° K.

Advantageously, thermocouples are inserted in the clusters of the coating material 4 for the control of the heat exchanges and the flow of the heat-transferring fluid.

The hydraulic connection between the various tubes 3 will depend on the flow rate of the heat-transferring fluid. The tubes 3, placed geometrically in parallel, may be hydraulically connected either in series or in parallel.

Preferably, the exchanger E is produced directly inside the duct 2, by placing the tubes 3 in the bottom of the duct and by pouring the coating material 4 in place in order to embed the tubes therein, the material closely matching the profile of the bottom portion of the duct. Prior to these operations, the flow of effluent in the duct 1 is diverted or stopped temporarily. As a variant, the exchanger E may be prefabricated; in this case its length is limited by the conditions of insertion into the duct.

The tubes 3 may be attached beforehand to a rack 6 (FIG. 7) formed of two half-shells 6 a, 6 b which are assembled to trap the tubes 3 in an evenly spaced configuration (FIG. 7), so as to maintain the spacing between each tube. The tubes may be attached by clips against the internal surface of the wall of the duct 1.

After the cluster of tubes 3 is installed in the duct, the coating material 4 is poured around the tubes and hardens in situ in order to maintain and protect the cluster of tubes 3. The coating material 4 forms a smooth exchange wall 5 between the effluent 2 and the heat-transferring fluid which travels through the tube.

A protective film 7 (FIG. 2), notably a sheet of plastic material, may be provided between the internal surface of the duct 2 and the exchanger E incorporated in this duct.

An intermediate webbing 8, notably made of metal or of synthetic fibers, is advantageously deployed (FIGS. 2 and 3), before the pouring of the coating material, over at least a portion of a cluster of tubes 3 in order to enhance the mechanical strength and/or to improve the thermal transfer when this webbing has a thermal conduction property, in particular when this webbing is metallic.

The coating of the tubes 3 may be carried out in several layers of material of different properties, namely:

-   -   one layer 9 of a thermally insulating material (FIG. 2) between         the tubes 3 and the wall of the duct 2, or the film 7 when such         a film is provided;     -   one layer of a thermally conductive material 10 in contact with         the tubes, between the tubes and the effluent, and     -   on the surface, a layer 11 of erosion-resistant material.

The flexible or semi-rigid tubes 3 are advantageously chosen to be able to be rolled up on a reel and unrolled over a considerable length, notably of several tens of meters. This makes it possible to produce an exchanger E, depending on the flow of the effluent, that has a sufficient length to make it possible to obtain the desired effectiveness, even though the coating of material 4 usually has a thermal conductivity that is less than that of the metals.

The tubes 3 may be made of flexible metal materials (copper, aluminum) or of synthetic materials (polypropylene, flexible plastic).

The profile of the tubes may be smooth or annulated as illustrated by the profile 12 in FIG. 8. The annulated profile makes it possible to increase the exchange surface area in contact with the coating and to provide a degree of compensation for differential expansion between the tubes 3 and the rigid coating material 4.

The number of tubes 3 present in an exchanger E is preferably at least equal to five, these tubes being evenly spaced.

The cross section of the tubes 3 may be circular or ovoid depending on the manufacturing method and the materials used.

Clusters of flexible tubes 3 may be prefabricated and rolled up on supports in the form of reels for being installed subsequently in the duct 1, by being unrolled from the reel.

A prefabrication at the factory of exchanger panels attached by bonding to a grid-like support or on a flexible film is also possible.

According to FIG. 4, the duct 2 comprises, in the bottom portion, a groove 13 in which the exchanger E obtained by pouring material 4 around the tubes 3 is installed. The groove 13, for example with a rectangular cross section, is filled by the exchanger E of which the top surface 5 remains horizontal.

According to the examples of FIGS. 2 and 5, the tubes 3 are distributed in an arc of the section of the duct 2 and the internal surface 5 a of the molded material is in an arc of a circle that is inwardly concave.

According to the embodiment of FIG. 6, the tubes 3 are placed along the complete contour of the cross section of the duct, on a circumference in the example shown, and the coating is also provided all around the internal surface of the duct so that a portion of the exchanger E is situated above the effluent. This top portion exchanges heat with the internal atmosphere of the duct 1.

According to the exemplary embodiment of FIG. 9, the installation for extracting heat from the effluent 1 comprises, in addition to the exchanger E which is immersed in the effluent, heat-exchanging tubes 14, arranged in a cluster, placed inside and in the top portion of the duct 2, outside the area in which the liquid effluent 1 flows. The tubes 14 are attached to areas of the internal wall of the duct that are situated above the effluent and are exposed to the atmosphere that prevails in the duct 2. The tubes 14 are travelled through by a heat-transferring fluid to recover a portion of the noticeable heat and of the latent heat from condensation of the water vapor originating from the effluent. The drops of condensed water, forming on the tubes, fall into the effluent as illustrated by arrows G.

The tubes 14 are oriented parallel to the longitudinal direction of the duct, and are connected in parallel or in series to form a fluid circuit usually comprising a heat pump. This fluid circuit is advantageously the same as that of the exchanger E.

The tubes 14 may be externally smooth or may comprise fitted fins to increase the exchange surface area. The fins may be spiraled, longitudinal, transverse, notably in a plane orthogonal to the axis of the tube, or annulated. According to FIGS. 9 and 10, the tubes 14 are of circular section and comprise fins 15 in the form of circular rings. According to FIG. 11, the tube 14 is of circular section, with oblong fins 15 a. According to FIG. 12, the tube 14 b is of oblong section with oblong fins 15 b.

The tubes 14 may be metallic or made of synthetic or plastic material. They may be rigid, semi-rigid or flexible. Advantageously the tubes 14 are suitable for being rolled up on a reel so that considerable lengths can be installed, notably of several tens of meters, in a single piece without connectors.

Placing the heat exchanger E inside a duct 1 has many advantages.

The exchanger fits all profiles of duct, ovoid, curvilinear and rectangular, and its length is unlimited when it is poured in place.

The tubes that are used are ordinary tubes that are low-cost and do not require the fabrication of special mechanical and dished parts.

The weight of the components is quite low such that they can be inserted without handling means into the duct 2.

The exchanger has a continuity over its length, with limited risk of leakage since it requires only a few hydraulic connectors over a great length, namely several tens of meters. The tube clusters may be connected by standard compression seals. The space requirement inside the ducts is reduced. No obstacle is created on the path of the effluent, in particular of the wastewater.

An on-site repair can be carried out requiring only a few handling means and toolage. In the event of an accidental breakage, a rapid repair can be carried out by sleeve and breastplate.

Very long flexible tubular panels can be inserted through a small orifice, notably a manhole.

The risk of corrosion of the synthetic materials or of the mineral coating or of the composite resin is low.

The invention may be applied to the recovery of heat from fluids that are laden and/or corrosive, abrasive, notably in urban sanitation systems.

The invention may also be employed in wastewater treatment works (STEP) between the upstream end and the downstream end of digesters, because the device is reversible and can transmit heat to an effluent that has to be heated up before a treatment.

The invention may be applied to any residential, tertiary or industrial utility that evacuates an effluent or warm effluents: swimming pools, schools, universities, administrative buildings, food, chemical or petrochemical industries.

The sealing of the installation may be inspected by pressurizing with compressed air and maintaining the pressure.

The balancing of the flow rate in each tube 3 may be carried out with the aid of calibrated diaphragms. In the event of an accidental leak from a tube, it is possible to block off this tube in order to take it out of the circuit. 

1.-15. (canceled)
 16. A method for extracting heat from an effluent flowing in a duct, notably a main sewer, according to which a heat exchanger is installed at least in the bottom of the duct, which heat exchanger is immersed in the effluent, the heat exchanger is formed by coating of tubes with a sufficiently heat-conducting material poured around the tubes, and suitable for hardening in situ, the tubes being designed for the flow of a heat-transferring fluid, the exchange of heat with the effluent of the duct taking place through the molded coating, the top surface of the poured material coming into direct contact with the effluent flowing in the duct, wherein the coating is carried out in several layers of materials with different properties, namely: a layer of thermally insulating material between the tubes and the wall of the duct, a layer of thermally conducting material in contact with the tubes, between the tubes and the effluent, and, on the surface, in contact with the effluent, a layer of abrasion-resistant material.
 17. The method as claimed in claim 16, characterized in that, after having diverted or temporarily stopped the flow of effluent in the pipe, the exchanger is implemented directly inside the duct, by depositing, in the bottom of the duct, tubes for a circuit of heat-transferring fluid, and by pouring in place the coating material in order to immerse the tubes therein, the material closely matching the profile of the bottom portion of the duct.
 18. The method as claimed in claim 16, wherein the coating material consists of at least one of the following materials: synthetic resin, thermally conductive cement or concrete, composite material optionally laden with thermally conductive additions or additives.
 19. The method as claimed in claim 16, wherein a protective film is inserted between the inner surface of the duct and the exchanger incorporated into this duct.
 20. The method as claimed in claim 16, characterized n that the tubes are placed parallel to the longitudinal direction of the duct.
 21. The method as claimed in claim 20, wherein the tubes, before pouring of the coating material, are kept parallel and spaced apart by a rack. Or a webbing fitted and bonded at the factory.
 22. The method as claimed in claim 16, wherein an intermediate webbing, made of metal or of synthetic fibers, is deployed before pouring of the coating material over at least a portion of a cluster of tubes in order to enhance the mechanical strength and/or improve the thermal transfer.
 23. The method as claimed in claim 16, wherein the tubes are flexible or semi-rigid in order to be able to be rolled up and unrolled continuously over a considerable length, notably of several tens of meters.
 24. The method as claimed in claim 23, wherein the tubes are made of flexible metal material or of synthetic material.
 25. The method as claimed in claim 16, wherein clusters of flexible tubes are prefabricated and that they are rolled up on reel-shaped supports for subsequent placement in the effluent duct by unrolling from the reel.
 26. The method as claimed in claim 16, wherein tubes for heat-transferring fluid are placed along the complete contour of the cross section of the duct, and that coating of the tubes is carried out over the whole contour of the section of the duct.
 27. The method as claimed in claim 16, wherein the internal surface of the molded material is horizontal.
 28. An exchanger for extracting heat from an effluent flowing in a duct, notably a main sewer, designed to be installed at least in the bottom of the duct and to be immersed in the effluent, the exchanger consisting of tubes embedded in a sufficiently heat-conducting material poured around the tubes that are intended for the flow of a heat-transferring fluid, the exchange of heat with the effluent of the duct taking place through the molded coating, to the exclusion of any fitted mechanical part, the upper surface of the poured material coming directly into contact with the effluent flowing in the duct, wherein the exchanger is produced according to a method as claimed in claim
 16. 29. An installation for extracting heat in an effluent duct, notably in a main sewer, wherein it comprises in the bottom, immersed in the effluent, at least one heat exchanger made according to the method of claim
 16. 30. The installation as claimed in claim 29, further comprising, in the areas of the internal wall of the duct that are situated above the effluent, pipes exposed to the atmosphere prevailing in the duct, through which pipes a heat-transferring fluid travels in order to recover a portion of the noticeable heat and of the latent heat from condensation of the water vapor. 